If you are not legally married then your GF is not your spouse. She might meet the criteria to be claimed as your dependent for the $500 credit for other dependents, but you cannot file a joint tax return with someone to whom you are not married.
https://ttlc.intuit.com/questions/4499708-what-is-the-500-credit-for-other-dependents-family-tax-cre...
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
